The present invention relates to an electronic control system for an automatic vehicle transmission, and more particularly to an electronic control system for controlling a vehicle transmission based on a vehicle speed, an amount of depression of an accelerator pedal, or the like.
FIG. 4 of the accompanying drawings schematically shows an automobile incorporating a conventional electronic control system 1 for automatically controlling a vehicle transmission.
Denoted in FIG. 4 at 2 is an engine, 3 a clutch, 4 a transmission, 5 a final drive unit, 6 a wheel, 7 a selector lever, and 8 an accelerator pedal.
Designated at 9 is an input shaft sensor for detecting the rotational speed of an input shaft of the transmission 4, 10 a vehicle speed sensor for detecting the speed of travel of the vehicle, 11 a gear position sensor for detecting a gear position of the transmission 4, and 12 an accelerator sensor for detecting the amount or depth to which the accelerator pedal 8 is depressed. Output signals from these sensors are supplied to the electronic control system 1. The clutch 3 and the transmission 4 are associated with a clutch actuator 13 and a transmission actuator 14, respectively, for mechanically operating the clutch 3 and the transmission 4. The clutch and transmission actuators 13, 14 are controlled by the electronic control system 1 based on the output signals from the sensors 9 through 12 and an output signal from the selector lever 7.
FIG. 5 shows in block form the electronic control system for explaining main operation thereof. The electronic control system 1 includes a pulse input unit 101 for counting output pulses from the vehicle speed sensor 10 and an analog input unit 102 for converting an analog output from the accelerator sensor 12 to a digital signal through A/D conversion. In a processor 103, a count from the pulse input unit 101 is put in a variable "SPEED", and a digital signal from the analog input unit 102 is put in a variable "ACCEL". The processor 103 searches a predetermined speed change map (MAP) for an optimum gear position based on "SPEED" and "ACCEL". The processor 103 then issues the optimum gear position to an output unit 104 which operates the clutch actuator 13 and the transmission actuator 14 for speed changing operation.
A processing sequence of the processor 103 will be described with reference to FIGS. 6(a) and 6(b). Variables used in this processing sequence are shown in FIG. 7.
A step a.sub.1 in FIG. 6(a) determines whether the present gear position is a neutral gear position or not. If the present gear position is other than the neutral gear position, then control goes to a process for detecting an abnormality of the vehicle speed sensor in steps a.sub.2, through a.sub.4. In this detecting process, a step a, first determines a variable "SPD:REAL" is smaller than a preset value (1.5 km/h in this example). If smaller than 1.5 km/h, then the vehicle speed sensor is determined as being abnormal in a step a.sub.3. If equal to or greater than 1.5 km/h, then the vehicle speed sensor is determined as being normal in a step a.sub.4.
Then, control goes to a step a.sub.5 to confirm the result of the above process for detecting an abnormality of the vehicle speed sensor. If the vehicle speed sensor is normal, then a variable "SPEED:REAL" is subjected to a filtering process and then put in a variable "SPD" in a step a.sub.6. If the vehicle speed sensor is abnormal, then a step a.sub.7 determines whether the gear position is the neutral position or not. If not the neutral position, control goes to a step a.sub.8. In the step a.sub.8 , the variable "SPD:REAL" is not used, but the vehicle speed is calculated from the rotational speed of the input shaft of the transmission and the gear ratio thereof, and put in "SPD".
In a next step a.sub.9, "SPD" obtained in the step a.sub.6 or a.sub.8 is put in the variable "SPEED". If the gear position is the neutral position in the step a.sub.7, control does not go through the step a.sub.9, and hence "SPEED" is retained as it is without being updated. In a step a.sub.10, an optimum gear position is determined from the speed change map based on "SPEED" and "ACCEL", and put in a variable "GEAR". In a next step a.sub.11, the value of "GEAR" is applied to the output unit 104 (FIG. 5) to operate the clutch actuator 13 and the transmission actuator 14.
In the above conventional electronic control system, when the vehicle speed sensor fails, if the gears are in any of gear positions other than the neutral gear position, it is possible to calculate the vehicle speed from the gear ratio and the input shaft rotational speed (step a.sub.8 in FIG. 6). Moreover, the proportion of the time in which the gears are in the neutral position to the entire running time of the automobile is very small. In view of these considerations, any abnormality of the vehicle sensor while the gears are in the neutral position has not been detected in the conventional electronic control system (steps a.sub.1 through a.sub.4).
It has been found, however, that when the vehicle speed sensor fails while the gears are in the neutral gear position, the vehicle as it runs may become dangerous with the aforesaid processing sequence. This dangerous situation will be described with reference to FIG. 8, which shows the relationship between the vehicle speed and time when the vehicle is running.
It is assumed here that the vehicle sensor fails at a time t.sub.10 after the selector lever is operated by the driver into a neutral gear position at a time t.sub.0. The processing of the electronic control system 1 now goes from the step a.sub.1 to the step a.sub.5 to the step a.sub.6 in FIG. 6. However, when the vehicle sensor fails, it does not issue vehicle speed pulses, and hence the variable "SPD:REAL" is "0". Therefore, "SPD" obtained in the step a.sub.6 approaches "SPD:REAL" (=0) at a certain gradient based on the filtering process as indicated by the dot-and-dash line, and then becomes "0" at a time t.sub.11. Thus, the electronic control system 1 determines that the vehicle is already at rest at the time t.sub.11 regardless of the fact that the actual vehicle speed (represented by the solid line) is still high.
Thereafter, at a time t.sub.2, when the driver shifts the selector lever into a drive position, the gears tend to be shifted into a second gear position irrespective of the fact that the actual vehicle speed a [km/h] is far in excess of a speed range (below b [km/h]) suitable for the second gear position. If the vehicle speed sensor were normal at this time, "SPD" would represent a [km/h], and the gears would not be shifted into the second gear position. However, since the vehicle speed sensor is abnormal, and hence "SPD"=0 [km/h] as described above, the gears are shifted into the second gear position inasmuch as this value of "SPD" is in the speed range suitable for the second gear position. As a consequence, the transmission 4 is caused to rotate excessively, and the engine is liable to overrun. If the engine overruns, no safety running of the vehicle is ensured, and the life of the driver may be endangered.